Predicting Old Faithful: The time to the next eruption, the interval, is predicted using the duration of the current eruption. The duration is timed from the first heavy surge which lifts water skyward at the start of the eruption until the last small splash above the cone at the very end. The longer the eruption lasts, the longer the interval until the next eruption. For instance, a 2 minute eruption results in an interval of about 55 minutes whereas a 4.5 minute eruption results in an interval of about 90 minutes.
It is not possible to predict more than one eruption in advance.

Duration

1.5 min

2.0 min

2.5 min

3.0 min

3.5 min

4.0 min

4.5 min

5.0 min

Interval

50 min

57 min

65 min

71 min

76 min

82 min

89 min

95 min

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